Coated films and coating compositions

ABSTRACT

There is described a substantially planar self-supporting sheet (preferably an thermoplastic film e.g. formed from a polyolefin such as BOPP) comprising on at least one side thereof a substantially continuous adherent coating thereon to create a barrier to oxygen, the coating comprising nano-particles, the sheet characterised in that: (a) the nano-particles form a substantially continuous phase on the sheet; and/or (b) the nano-particles are present in an amount of at least about 20% by dry weight of the coating; and/or (c) the coating is substantially free of polymeric binder, synthetic hydrophilic resin and/or a resin with a hydrophilic/hydrophobic character. The coated sheets have a high oxygen barrier, with preferred OTR values (at 23° C. and 0% RH)&lt;10 cm 3 /m 2 /day. Preferred nano-particles are biopolymers such as starch and the coating is preferably substantially free of wax which has been found to reduce oxygen barrier effectiveness. Coatings components of small molecules or oligomers such as biomonomers (e.g. sucrose) have also been found to enhance oxygen barrier properties of a particulate coat whether the particles are micrometer or nanometre sized. Coating compositions, coated films, coating processes and packaged articles are also described.

[0001] The present invention relates to improved coating compositionsand films coated with such compositions, the coated films havingproperties such as a low permeability to gases (high gas barrier).

[0002] One important use of films is in the packaging of perishableitems such as food. It is desirable in this and other applications thatpackaging film provides a good barrier to gases such as oxygen. Howevermany commonly used film substrates such as polyolefins (e.g.polypropylene) do not provide good gas barrier properties. Therefore itis often desirable to add various coatings to a film to enhance orprovide desired properties depending on the end use of the film.

[0003] Current coatings used to provide an improved oxygen barrier to afilm comprise materials such as polyvinylidene chloride (PVdC). Howeverthis material is expensive and as it contains halogen is undesirable forenvironmental reasons. For example films coated with PVdC are moredifficult to recycle. If PVdC coated film is incinerated suitable meansmust be provided to eliminate the generation of dioxins. Chlorinecontaining packaging materials (such as PVdC coated film) areconsequently less favoured in some countries.

[0004] Therefore it is desirable to find alternative gas-barriercoatings which solve some or all of the properties of prior art coatingsand preferably which give comparable or improved properties to the filmcompared to conventional coatings.

[0005] Starch dispersions have previously been used as coatingcompositions, e.g. to coat paper. Starch dispersions refer to aqueousstarch systems where the granular starch structure has partly or totallybeen disrupted using sufficient work or heat. In excess of water, theprocess of starch hydration and granule destruction is known asgelatinisation and can be facilitated or impeded by chemicalmodifications (cf. converted, derivatised or cross-linked starch)(“Starch: Chemistry and Technology”, R. L. Whistler, J. N. BeMiller, E.F. Paschall Eds., Academic Press, London, 1984). Pre-gelatinisation andphysical treatment (e.g. via extrusion, drum- or spray drying) of starch(and derivatives) yields cold-water dispersible systems. Although thereis no need for cooking, pre-gelatinised starch still requiressubstantial mechanical energy for dispersion in water. U.S. Pat. No.5,032,683 describes the production of a stable aqueous starchcomposition having a coatable viscosity, wherein the starch has beengelatinised and reacted at temperatures above 70° C. in the presence ofa blocked glyoxal compound. However, converted (thin-boiling) and/orchemically modified starch are the advised starch materials for thesecompositions. U.S. Pat. No. 5,116,890 describes the preparation ofwater-dispersible, self cross-link lattices based on a starchhydrolysate to produce stable emulsions with low viscosity. The processrequires undesirable chemicals and is quite laborious.

[0006] Blends of starch and synthetic polymers obtained viathermoplastic processing, are also well-known in the art (“Production ofThermo-Bioplastics and Fibers based mainly on Biological Materials”,H.-G. Fritz, T. Seidenstücker, U. Bölz and M. Juza, EU-StudyAGRE-0200-DE, 1994). These materials are usually developed for theproduction of moulded or shaped articles, not for the preparation oflatex compositions for coating purposes. Numerous patents describe thedestructure of starch in combination with hydrophilic and hydrophobicsynthetic components, such as vinyl alcohol copolymers (EP 327505, EP408503), polyvinylesters and polyesters (EP 327565, U.S. Pat. No.5,439,953), aliphatic polyesters (WO 92/19680), polyolefins (WO92/20740), ethylene acrylic acid (EM) copolymer (U.S. Pat. No.4,133,784, U.S. Pat. No. 5,262,458). U.S. Pat. No. 5,262,458, describesa starch/EAA blend where after processing part of the destructuredstarch (<30% of total starch in the mixture) comprises particles withdiameter lower than 1 micrometer.

[0007] A co-pending application PCT/NLOO/00050 by ATO-DLO (published on23 Nov. 2000 after the priority date of the present application)describes a new thermo-mechanical process for the production ofcross-linked biopolymer (such as starch) using extrusion at raisedtemperatures and under conditions of high shear and high pressure. Thisthermo-mechanical treatment is conducted in the presence of across-linking agent and preferably a plasticiser is also present. Thestarch material obtained can readily be dispersed in cold water atincreased solids content (up to 40% by weight), and has relatively lowviscosity (typically lower than 100 mPa.s at 25° C. and 10% by weight)compared to other cold-water dispersible starches. The average size ofthese dispersed starch particles is in the submicron size range,typically less than 200 nm. One of over twenty different possible useslisted for such nano-particles is to provide an high barrier coating foroxygen, although films are not mentioned as a possible substrate. Thisdocument has no more than a passing reference to use of these particlesin an oxygen barrier coating and no specific coating formulations orcoated films are described therein. There is no disclosure in thisdocument of how the nanoparticles may be formulated in practise to coatfilms which may exhibit high oxygen barrier properties.

[0008] The applicant's co-pending patent application PCT/EP00/10503describes dispersions of these new nano-sized biopolymers (e.g. starch)in combination with synthetic hydrophilic resins and/or resins with ahydrophilic/hydrophobic character to impart the necessary stability. Oneuse of such dispersions is stated to be as coatings with a high oxygenbarrier, although this reference also states that in the absence ofthese resin additives, the nano-starch is unsuitable for coatingapplications. This leads a skilled reader directly away from the presentinvention which uses nano-starch to form a coating on a film where suchsynthetic resins are not an essential element of the coating. No coatedfilms are described in this document.

[0009] As well as being inventive the specific films and coatingcompositions of the present invention are clearly novel with respect tothe separate disclosures in either of the previous two unpublishedco-pending applications.

[0010] WO 00/40404 (Mobil) describes thermoplastic films which arecoated with a composition comprising a polymeric binder within which aredispersed nano-sized particles. Such coatings are said to act as abarrier for water and oxygen. The specific nano-particles describedtherein comprise inorganic material such as silicate clays and are addedto the coating in the form of a thermoplastic additive compositions suchthat the coating contains no more than 18% by weight of nano-particles.

[0011] The applicant has developed improved coating compositions andcoated films which comprise nano-sized particles (preferably ofnano-sized biopolymer). The applicant has discovered methods ofpreparing films with such coatings such that the film has the desiredbarrier properties. The absence and/or presence of certain ingredientsin preferred coatings of the invention has unexpectedly been found tolead to improved oxygen barrier properties.

[0012] Therefore broadly in accordance with the present invention thereis provided a substantially planar self-supporting sheet comprising onat least one side thereof a substantially continuous adherent coatingthereon to create a barrier to oxygen, the coating comprisingnano-particles, the sheet characterised in that:

[0013] (a) the nano-particles form a substantially continuous phase onthe sheet; and/or

[0014] (b) the nano-particles are present in an amount of at least about20% by dry weight of the coating; and/or

[0015] (c) the coating is substantially free of a polymeric binder,synthetic hydrophilic resin and/or a resin with ahydrophilic/hydrophobic character.

[0016] Conveniently the coating of the present invention comprises thenano-particles as substantial proportion thereof; preferably in amountof at least about 90%, more preferably about 95%; most preferably atleast about 98% by weight of the dry coating. It is preferred that suchproportions of nano-particles in the coating exclude any amounts of gasbarrier improving additives that may be added, but include only therelative proportion by weight in the coat of nano-particles with respectto other (non gas-barrier improving) additives. It is especiallypreferred that the coating of the present invention is substantiallyfree of any additives which are detrimental to the gas barrierproperties of the coating.

[0017] Conveniently the coating is substantially free of any componentswhich act to reduce the effectiveness of the coating as an oxygenbarrier.

[0018] Preferably a coated sheet of the invention has an oxygen transferrate (OTR measured at 23° C. and 0% relative humidity) of less thanabout 10 cm³/m²/day, more preferably less than about 5 cm³/m²/day; morepreferably less than about 3 cm³/m²/day.

[0019] It will be appreciated that the coatings of the invention use amaterial which has advantages over prior art coatings, such as one ormore of the following: kinder to the environment; use of a renewalresource; and/or more cost effective than prior art coatings (e.g. PVOH,EVOH or PVdC). Coatings of the present invention are preferably chlorinefree, more preferably halogen free, can be used as a replacement forprior art barrier coatings (such as PVdC) to prepare films havingcomparable OTR values to the prior art. But the coatings of theinvention have an improved OTR for a given coat weight compared to PVdCand so (especially when in combination with the oxygen barrier improvingadditives described herein) films having greatly improved OTR valuesalso can be prepared. Thus particularly preferred films of the inventionhave an OTR less than about 1 cm³/m²/day; in particular less than about0.5 cm³/m²/day.

[0020] As used herein the term “nano-particle” and/or “nano-sizedparticle” denote particles whose mean size is less than about 500 nm(0.5 microns). Preferably the nano-particles used herein have a meanparticle size less than about 300 nm; more preferably from about 5 nm toabout 300 nm; most preferably from about 10 nm to about 200 nm; forexample from about 10 nm to about 100 nm; in particular from about 20 nmto about 80 nm; e.g. about 50 nm.

[0021] Particle size as measured and described herein denotes in alinear dimension [e.g. measured in nanometres (1 nanometre≡1 nm≡1mμ≡1×10⁻⁹ m) which is the mean diameter of particles assuming anapproximate spherical shape or the mean length of the major axis forparticles of non-spherical shape. Preferably the particle sizedistribution is unimodal, narrow, substantially symmetrical and/orGaussian, although it could also be for example bimodal and/or skewed.More preferably the nano-particles used herein comprise no more thanabout 30%, most preferably no more than about 10% of particles having asize greater than about one micron (1 micron≡1 μ≡1 μm≡1×10⁻⁶ m).

[0022] Particle size herein can be determined by any convenienttechnique. For example dynamic light scattering (DLS) can be a usefultool to measure the average size and the size distribution of theparticles in a coating of the present invention. Light scatteringmeasurements can be performed at 25° C. on a MALVERN Autosizer Io-Cinstrument equipped with a 8-bit correlator. If a tolerable scatteredlight signal can be measured, samples can be diluted with de-mineralisedwater to a concentration lower than 0.1% by weight. At higherconcentrations, it can be verified that multiple scattering andconcentration (interparticle interactions) effects are insignificant forthe estimated particle sizes. The time-correlation function of thescattered light intensity may be recorded at an angle of 90° (usually intriplicate). The intensity correlation data may be analysed with theso-called “CONTIN” software package (S. W. Provencher, Comput. Phys.Commun. 27, 1982) to assess particle size distributions and the averageparticle size. Diameter estimation from DLS data is inherentlyintensity-weighted (i.e. weighted to the sixth power of the diameter).Preferably the particle size is measured shortly after the particles aremade, for example using the methods described herein.

[0023] Conveniently the nano-particles used herein are formed frommaterials other than: silica, silicate, clay, organomontmorillonite,calcium carbonate, calcined aluminium silicate, hydrated aluminiumsilicate, calcium phosphate, alumina, barium sulphate, magnesiumsulphate and/or diatomacious earth.

[0024] Both coatings of the present invention and formulations used toprepare coated sheets of the present invention (b.g. aqueous dispersionsof nano-particles) may conveniently comprise other than a thermoplasticand/or thermoset additives and are preferably substantially free of anysynthetic thermoplastic and/or thermoset binder resins. The coatings andformulations of the present invention may also comprise at least about20% (preferably ≧ about 90%) of the nano particles by weight.

[0025] The nano-particles used to form the films and coatings of thepresent invention may comprise (advantageously are formed substantiallyfrom) one or more organic polymers, preferably biopolymers, morepreferably selected from carbohydrates; polysaccharides (such as starch,cellulose, glycogen, hemi-cellulose, chitin, fructan inulin; ligninand/or pectic substances); gums; proteins, optionally cereal, vegetableand/or animal proteins (such as gluten [e.g. from wheat], whey protein,and/or gelatin); colloids (such as hydro-colloids, for example naturalhydrocolloids, e.g. gums); effective mixtures thereof; and/or effectivemodified derivatives thereof. More conveniently, the biopolymercomprises native and/or modified starch obtained and/or obtainable fromone or more plant(s); most conveniently a starch, starch-ether,starch-ester and/or oxidised starch obtained and/or obtainable from oneor more root(s), tuber(s) and/or cereal(s), preferably obtained and/orobtainable from potato, waxy maize, tapioca and/or rice.

[0026] Gluten may comprise a mixture of two proteins, gliadin andglutenin whose amino acid composition may vary although glutamic acidand proline usually predominate.

[0027] Gums are natural hydro-colloids which may be obtained from plantsand are typically insoluble in organic solvents but form gelatinous orsticky solutions with water. Gum resins are mixtures of gums and naturalresins.

[0028] As used herein the term carbohydrate will be understood tocomprise those compounds of formula C_(x)(H₂O)_(y), which may beoptionally substituted. Carbohydrates may be divided into saccharides(also referred to herein as sugars) which typically may be of lowmolecular weight and/or sweet taste and/or polysaccharides whichtypically may be of high molecular weight and/or high complexity.

[0029] Polysaccharides comprise any carbohydrates comprising one or moremonosaccharide (simple sugar) units. Homopolysaccharides comprise onlyone type of monosaccharide and heteropolysaccharides comprise two ormore different types of sugar. Long chain polysaccharides may havemolecular weights of up to several million daltons and are often highlybranched, examples of these polysaccharides comprise starch, glycogenand cellulose. Polysaccharides also include the more simple disaccharidesugars, trisaccharide sugars and/or dextrins (e.g. maltodextrin and/orcyclodextrin).

[0030] It is preferred that if polysaccharides comprise the biopolymersused herein, then the polysaccharides comprise a polymer of at leasttwenty or more monosaccharide units and more preferably have a molecularweight (M_(W)) of above about 5000 daltons. It will be appreciated thatthe less complex polysaccharides (such as oligosaccharides, disaccharidesugars, trisaccharide sugars, maltodextrins and/or cyclodextrins) may bemore suitable for use as barrier improving additives in the coatings ofthe present invention rather than forming the nanoparticles themselves.This is described more fully later on.

[0031] Non-limiting examples of more complex polysaccharides are listedbelow and one or more of these polysaccharides may be used as barrierimproving additive and/or biopolymer in the present invention asappropriate.

[0032] Starch (which occurs widely in plants) may comprise variousproportions of two polymers derived from glucose: amylose (comprisinglinear chains comprising from about 100 to about 1000 linked glucosemolecules) and amylopectin (comprising highly branched chains of glucosemolecules).

[0033] Glycogen (also known as animal starch) comprises a highlybranched polymer of glucose which can occur in animal tissues.

[0034] Cellulose comprises a long unbranched chain of glucose units.

[0035] Chitin comprises chains of N-acetyl-D-glucosamine (a derivativeof glucose) and is structurally very similar to cellulose.

[0036] Fructans comprise polysaccharides derived from fructose which maybe stored in certain plants.

[0037] Inulin comprises a polysaccharide made from fructose which may bestored in the roots or tubers of many plants.

[0038] Lignin comprises a complex organic polymer that may be depositedwithin the cellulose of plant cell walls to provide rigidity.

[0039] Pectic substances such as pectin comprise polysaccharides made upprimarily of sugar acids which may be important constituents of plantcell walls. Normally they exist in an insoluble form, but may changeinto a soluble form (e.g. during ripening of a plant).

[0040] Polylactic and/or polygalactic polymers and the like comprisethose polymeric chains and/or cross-linked polymeric networks which areobtained from, obtainable from and/or comprise: polylactic acid;polygalactic acid and/or similar polymers and which may be madesynthetically and/or sourced naturally.

[0041] Other types of polysaccharide derivatives one or more of whichmay also be used to form (in whole or in part) nanoparticles as used inthe present invention may comprise any effective derivative of anysuitable polysaccharide (such as those described herein) for examplethose derivatives selected from, amino derivatives, ester derivatives(such as phosphate esters) ether derivatives; and/or oxidisedderivatives (e.g. acids).

[0042] Advantageously the biopolymer particles are prepared by amechanical thermoplastic process. Thermoplastic processing as used inthis context means a thermo-mechanical treatment, which is in particularan extrusion treatment performed at elevated temperature (preferablyabove about 40° C., more preferably up to about 140° C.) underconditions of high shear and high pressure (preferably from about 5 toabout 150 bar). The shear can be effected by applying at least about 500J of specific mechanical energy per gram of biopolymer. The elevatedtemperature can be moderated, in case of starch, by using an alkalinemedium or by using pre-gelatinised starch. During the thermo-mechanicaltreatment, the biopolymer may be present in high concentration,especially a weight concentration of at least 40%, more preferably atleast 50%, in an aqueous solvent, such as water or a water/alcoholmixture.

[0043] It is preferred that a plasticiser is present during thethermoplastic process described above. The plasticiser may be selectedfrom one or more: polyols, ethyleneglycol, propyleneglycol, polyglycol,glycerol, sugar alcohols, urea and/or citric acid esters. Theplasticiser may be present in an amount from about 5% to about 40% byweight of the biopolymer. A lubricant, such as lecithin, otherphospholipids and/or monoglycerids, may also be present, for example inan amount from about 0.5% to about 2.5% by weight of the biopolymer.

[0044] Particularly preferred nano-particles of biopolymer for use inthe films and compositions of the present invention are those where thebiopolymer has been cross-linked for example during thethermo-mechanical treatment described herein. Any convenient, effectivecross-linker can be used, but usefully cross-linkers such asepichlorohydrin and other epoxides, dialdehydes (e.g. glutaraldehyde,glyoxal), triphosphates and/or divinyl sulphone, can be used tocross-link polysaccharide biopolymers, while dialdehydes, thiol reagentsand the like may be used to cross-link proteinaceous biopolymers.Glyoxal is a particularly suitable cross-linker. The cross-linkingreaction may be acid- or base catalysed. The cross-linking agent canconveniently be present in an amount from about 0.1% to about 10% byweight of the biopolymer.

[0045] After the thermo-mechanical treatment, the biopolymer can readilybe dissolved or dispersed in an aqueous medium to a concentration fromabout 4% to about 40% by weight. This results in a dispersion of starchnano-particles, which are characterised by an average particle size asspecified herein, which is preferably less than about 200 nm.

[0046] Preferably the coating comprises substantially none (or less thana gas barrier reducing amount) of any component which is capable ofsubstantially reducing the gas barrier (preferably oxygen barrier)properties of the coatings of the present invention when applied to afilm.

[0047] Preferably gas barrier inhibiting components which are to besubstantially excluded from the coatings of the present invention maycomprise one or more of those organic compounds and/or polymers whichcomprise linear carbon chains of greater than 10 carbon atoms,preferably C₁₆₋₄₄chains, more preferably natural or synthetic waxes.

[0048] The total amount of gas barrier inhibiting component(s) which canact to substantially reduce the oxygen barrier effectiveness of thenano-particle coat of the present invention may be a variable amountdepending on the undesirable ingredient. Conveniently preferred films ofthe invention are those coated with a nano-particle composition which isabout 90% free, more preferably about 95% more preferably about 98% freeof gas barrier inhibiting components (such as any of those describedherein). It will be appreciated that for maximum oxygen barriereffectiveness coatings of the invention may be substantially completelyfree of such components. However as a compromise if other properties aredesired in the coating some otherwise undesirable ingredients may beadded in small amounts just sufficient to improve and/or impart thedesired properties to the coating and/or film. For example some waxes(preferably no more than about 5%) may be added to a gas barrier coatingof the invention for other reasons without destroying the gas barrierproperties to a significant extent.

[0049] Preferably those gas barrier inhibiting component(s) to beavoided are natural or synthetic wax(es), especially if present in atotal amount greater than or equal to about 10% by weight of thecoating. Up to about 5% preferably up to about 4%, more preferably about2% of wax may be acceptable. Particularly undesirable in the coating ofthe invention are 10% or more by weight of waxes with a melting pointfrom about 50° C. to about 150° C. such as montan ester waxes (m.p. fromabout 60° C. to about 90° C.); carnauba waxes (m.p. from about 60° C. toabout 90° C.); and/or polyethylene waxes (m.p. about 130° C.).

[0050] Preferably a coating of the present invention also comprises anoxygen barrier improving amount of a component which acts to improve theoxygen barrier properties of the nano-particle coat when applied to afilm.

[0051] Gas barrier improving components may comprise those having one ormore of the following selected properties:

[0052] small polymers and/or oligomers having a short chain length suchthan the mean number of repeat units in the oligomer and/or polymerchain is less than or equal to about 20;

[0053] mean molecular weight (M_(W)) less than about 4,000; preferably <about 1,000, more preferably less than about 500 and for example about350 daltons;

[0054] chemical and/or physical compatibility with the particles in thecoat to aid close packing thereof;

[0055] components having combinations more than one of these propertiesin the same component; and/or

[0056] mixtures of different components with at least one of theseproperties.

[0057] The total amount of gas barrier improving component(s) which canbe added to the coating composition substantially to improve the gas(preferably oxygen) barrier effectiveness of the nano-particle coat maybe significant compared to the amount of nano-particles. Without wishingto be bound by any theory it is believed that in general up to an upperlimit the more of the gas-barrier improving additive that is added tothe coat the better the close packing of the particles and thus thebetter the gas barrier. Effective total amounts of gas barrier improvingingredients may therefore be from about 10% to about 99%, preferablyfrom about 20% to about 95%, more preferably from about 40% to about 90%by weight of the total coating.

[0058] Barrier improving components to be preferred (especially when theparticles in the coating comprise biopolymer) comprise those constituentmonomers and/or oligomers from which biopolymers may be made(biomonomers). Barrier improving components may comprise suitable bioderived monomers, dimers, trimers and/or oligomers. Conveniently thebarrier improving ingredient may comprise: dextrins (such asmaltodextrins and/or cyclodextrins); low molecular weight hydroxycompounds (such as glycerol); low molecular weight carbohydrates (suchas saccharides and/or sugars) and/or any effective mixtures and/orcombinations thereof.

[0059] Saccharides comprise monosaccharides (also known as simplesugars) and/or polysaccharides (some of which are also known as complexsugars). Complex sugars typically comprise from two to twenty(preferably from 2 to 10) monosaccharide units linked together (such asdisaccharides, trisaccharides, maltodextrins and/or cyclodextrins). Itwill be appreciated that one or more of any simple and/or complex sugar(optionally substituted) may be used as barrier improving additives inthe present invention.

[0060] Monosaccharides (or simple sugars) comprise carbohydrates thatcannot be split into smaller units by the action of dilute acids.Monosaccharides may be classified according to the number of carbonatoms they possess (such as trioses, tetroses, pentoses, hexoses etc);may also be characterised into aldoses (sugars comprising a aldehydegroup) and ketoses (sugars comprising a ketone group); can exist aseither straight-chain or ring-shaped molecules; and can also exist indifferent enantiomeric forms. Monosaccharides can be oxidised to yieldsugar acids and react with phosphoric acid to produce phosphate esters.

[0061] Non-limiting examples of simple sugars are listed below; and oneor more of these sugars may be used as gas-barrier improving additivesin the present invention.

[0062] Fructose (also known as fruit sugar; laevulose) is a ketohexose,a stereoisomer of glucose, which occurs naturally as the d-enantiomer.

[0063] Furanose has a five-membered ring containing four carbon atomsand one oxygen atom.

[0064] Galactose occurs naturally as one of the products of the enzymicdigestion of milk sugar (lactose) and is a constituent of gum arabic.

[0065] Glucose (also known as dextrose or grape sugar) is a whitecrystalline aldohexose of formula C₆H₁₂O₆, occurring widely in naturewhich is optically active like other monosaccharides (most naturaloccurring glucose being dextrorotatory).

[0066] Mannose is a stereoisomer of glucose, that occurs naturally onlyin polymers called mannans found in plants, fungi, and bacteria.

[0067] Mannitol is a polyhydric alcohol, CH₂OH(CHOH)₄CH₂OH, derived frommannose or fructose.

[0068] Pyranose has a six-membered ring containing five carbon atoms andone oxygen atom.

[0069] Non limiting examples of disaccharides are listed below, and oneor more of these sugars also may be used as barrier improving additivesin the present invention.

[0070] Lactose (also known as milk sugar) comprises one glucose moleculelinked to a galactose molecule.

[0071] Maltose (also known as malt sugar) comprises two linked glucosemolecules and results from the action of the enzyme amylase on starch.

[0072] Sucrose (also known as cane sugar, beet sugar or saccharose)comprises one molecule of glucose linked to a fructose molecule.

[0073] Non limiting examples of other polysaccharides (such asoligosaccharides) are listed below, and one or more of these also may beused as barrier improving additives in the present invention.

[0074] Maltodextrin comprises dextrins of up to about twenty glucoseunits and has a typical M_(W) of about 3,600.

[0075] Cyclodextrin comprises dextrins of about eight glucose unitswhich together form a ring.

[0076] Other types of derivatives of the above compounds one or more ofwhich may also be used as gas-barrier improving additives in the presentinvention comprise: amino sugars; sugar esters (such as phosphateesters); sugar ethers; and/or oxidised sugar derivatives (e.g. sugaracids). Amino sugars comprise a sugar containing an amino group in placeof a hydroxyl group, for example hexosamines are amino derivatives ofhexose sugars and include glucosamine (based on glucose) andgalactosamine (based on galactose).

[0077] Preferred sugars and sugar derivatives used as gas-barrierimproving additives in the present invention are those compounds havingfrom one to twenty (inclusive), more preferably one to eightmonosaccharide units per molecule. Mono- and/or di-saccaharides; aremost preferred such as fructose, glucose and/or sucrose.

[0078] Advantageously the barrier improving component may be present inthe form of an encapsulate for example within a miscelle formed bypolymer chains. Such encapsulates may be usefully formed by anyconvenient method such as spray or freeze drying of the appropriateingredients.

[0079] Without wishing to be bound by any mechanism it is believed thatthe barrier improving component acts to aid packing of the particles inthe coating giving a more compressed coating layer which acts as abetter barrier to oxygen molecules. For example components of shortchain length (e.g. sucrose) more really fill the interstices and voidsbetween the particles (e.g. biopolymer nano-particles).

[0080] Although nano-particles are preferred in combination with thebarrier improving component it can be seen from the data herein thatbarrier improving components can also be used in compositions havinglarger particles (greater than nano sized) to improve the oxygen barrierproperties.

[0081] Therefore broadly in accordance with another aspect of thepresent invention there is provided a substantially planar selfsupporting sheet comprising on at least one side thereof a substantiallycontinuous adherent coating thereon to create a barrier to oxygen, thecoating comprising nano and/or micro-sized particles; the sheetcharacterised in that: the coating further comprises, in an gas-barrier(preferably oxygen-barrier) improving amount, a gas-barrier improvingcomponent (such as any of those described herein) which acts to enhancethe gas barrier properties of the coating. Optionally particles of justnano-size are preferred as are those particles which comprise biopolymersuch one or more of those described herein.

[0082] As used above the term “micro-particle” denote particles whosemean size is from about 1 micron to about 100 microns and preferably hasless than about 20% of the particles of a size below about one micron.Preferred particles used in this aspect of the present invention arethose of nano-size, such as described herein.

[0083] Unless the context indicates otherwise, the terms ‘effective’and/or ‘suitable’ as used herein (for example with reference to thesheets, films, coatings, formulations, process, methods, uses,applications, products, materials, additives, compounds, monomers,oligomers, polymer precursors, polymers and/or resins described hereinand/or used in, added to and/or incorporated in the present invention)will be understood to refer to those components which if used in thecorrect manner provide the required properties (such as an improvedgas-barrier) to the present invention as described herein.

[0084] It will also be understood that any optional substituents thatmay be present on any repeat unit in any polymer described herein may beselected to improve the compatibility thereof with any other materialswith which they may be formulated and/or incorporated to form theinvention herein.. Thus, the size and length of substituents may beselected to optimise the physical entanglement or interlocation with theresin or they may or may not comprise other reactive entities capable ofchemically reacting and/or cross-linking with such resins.

[0085] Certain moieties, species, groups, repeat units, compounds,oligomers, polymers, materials, mixtures, compositions and/orformulations which comprise some or all of the invention as describedherein may exist as one or more stereoisomers (such as enantiomers,diastereoisomers, geometric isomers, tautomers and/or conformers),salts, zwitterions, complexes (such as chelates, clathrates, crowncompounds, cyptands/cryptades, inclusion compounds, intercalationcompounds, interstitial compounds, ligand complexes, non-stoichiometriccomplexes, organometallic complexes, π-adducts, solvates and/orhydrates); isotopically substituted forms, polymeric configurations[such as homo or copolymers, random, graft or block polymers, linear orbranched polymers (e.g. star and/or side branched polymers),hyperbranched polymers and/or dendritic macromolecules (such as those ofthe type described in WO 93/17060), cross-linked and/or networkedpolymers, polymers obtainable from di and/or tri-valent repeat units,dendrimers, polymers of different tacticity (e.g. isotactic,syndiotactic or atactic polymers)]; polymorphs [such as interstitialforms, crystalline forms, amorphous forms, phases and/or solidsolutions] combinations thereof where possible and/or mixtures thereof.The present invention comprises and/or incorporates all such forms whichare effective and/or suitable.

[0086] It is appreciated that certain features of the invention, whichare for clarity described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Converselyvarious features of the invention, which are for brevity, described inthe context of a single embodiment, may also be provided separatelyand/or in any suitable sub-combination.

[0087] The term “comprising” as used herein will be understood to meanthat the list following is non-exhaustive and may or may not include anyother additional suitable items, for example one or more furtherfeature(s), component(s), ingredient(s) and/or substituent(s) asappropriate.

[0088] The sheet used in the present invention, prior to deposition ofany coating (e.g. the gas barrier-coating of the present invention)and/or layer may be any suitable substrate, such as any well knownsheeting material(s). Suitable sheeting materials may comprise any ofthe following: paper, synthetic paper, woven fabric, non-woven fabric,ceramic sheet, metallic fibre sheet, metallised sheet or film, metallicfoil, metallic plate; films made from biopolymers such as any of thosedescribed herein [preferably polylactic, polygalactic and/or cellulosicfilms (e.g. microbal and/or regenerated cellulose film)]; thermoplasticfilms; polymeric films (for example films comprising: polyolefins [e.g.polypropylene and/or polyethylene] polyurethanes, polyvinylhalides [e.g.PVC], polyesters [e.g. polyethylene terephthalate—PET], polyamides [e.g.nylons] and/or non-hydrocarbon polymers); and/or multilayer and/orcomposite sheets formed by any suitable combinations and/or mixtures ofthereof. Advantageously sheets of the present invention are other thanpaper and/or are chlorine-free, more advantageously halogen free.

[0089] Preferably the sheet comprises a cellulosic material, polymericmaterial and/or thermoplastic polymer, conveniently comprising polymersof low surface energy. More preferably the sheet comprises ahomopolymer, a crystalline polymer and/or a polymer of randomly orientedamorphous non-crystalline polymer chains. Most preferably the sheetcomprises: polyolefins [e.g. polypropylene and/or polyethylene]polyurethanes, polyvinylhalides [e.g. polyvinyl chloride (PVC)],polyesters [e.g. polyethylene terephthalate—PET], polyamides [e.g.nylons] and/or non-hydrocarbon polymers).

[0090] Conveniently the polyolefin films to be used with the presentinvention may comprise one or more polyolefins [e.g. polypropylenehomopolymer, polyethylene homopolymer (e.g. linear low-densitypolyethylene—LLDPE) and/or polypropylene/polyethylene copolymer(s);optionally in one or more layers]. The constituent polymers and/orlayers in a film of the present invention may be oriented, blown,shrunk, stretched, cast, extruded, co-extruded and/or comprise anysuitable mixtures and/or combinations thereof. Preferred films comprisea major proportion of polypropylene and/or an olefin block copolymercontaining up to about 15% w/w of the copolymer of at least onecopolymerisable olefin (such as ethylene). More preferred films comprisepolypropylene homopolymer, most preferably isotactic polypropylenehomopolymer. Films may optionally be cross-linked by any suitable meanssuch as electron beam (EB) or UV cross-linking, if necessary by use ofsuitable additives in the film.

[0091] The definition of polyolefin, as intended herein, is a polymerassembled from a significant percentage, preferably ≧50% by weight ofone or more olefinic monomers. The definition of copolymer herein, is apolymer assembled from two or more monomers. Such polymers may include,but are not limited to, polyethylene homopolymers, ethylene-α-olefincopolymers, polypropylene-α-olefin copolymers, polypropylenehomopolymers, ethylene-vinyl acetate copolymers, ethylene-methacrylicacid copolymers and their salts, ethylene-styrene polymers and/or blendsof such polymers. The polymers may be produced by any suitable means,for example one or more of free radical polymerisation (e.g. peroxycompounds), metallocene catalysis and/or coordination catalysis (e.g.Ziegler and/or Natta catalysts and/or any variations thereof).

[0092] Polymeric resins used to produce the films of the presentinvention are generally commercially available in pellet form and may bemelt blended or mechanically mixed by well-known methods known in theart, using commercially available equipment including tumblers, mixersand/or blenders. The resins may have other additional resins blendedtherewith along with well-known additives such as processing aids and/orcolorants. Methods for producing polyolefin films are well-known andinclude the techniques of casting films as thin sheets through narrowslit dies, and blown-film techniques wherein an extruded tube of moltenpolymer is inflated to the desired bubble diameter and/or filmthickness.

[0093] For example to produce a polymeric film the resins and additivesmay be introduced into an extruder where the resins are melt plastifiedby heating and then transferred to an extrusion die for formation into afilm tube. Extrusion and die temperatures will generally depend upon theparticular resin being processed and suitable temperature ranges aregenerally known in the art or provided in technical bulletins madeavailable by resin manufacturers. Processing temperatures may varydepending upon process parameters chosen.

[0094] A film of the present invention may be oriented by stretching ata temperature above the glass transition temperature (T_(g)) of itsconstituent polymer(s). The resultant oriented film may exhibit greatlyimproved tensile and stiffness properties. Conveniently a filmcomprising a propylene homopolymer is oriented at a temperature within arange of from about 145° to 165° C. Orientation may be along one axis ifthe film is stretched in only one direction, or may be biaxial if thefilm is stretched in each of two mutually perpendicular directions inthe plane of the film. A biaxial oriented film may be balanced orunbalanced, where an unbalanced film has a higher degree of orientationin a preferred direction, usually the transverse direction.Conventionally the longitudinal direction (LD) is the direction in whichthe film passes through the machine (also known as the machine directionor MD) and the transverse direction. (TD) is perpendicular to MD.Preferred films are oriented in both MD and TD.

[0095] Orientation of the film may be achieved by any suitabletechnique. For example in the bubble process the polypropylene film isextruded in the form of a composite tube which is subsequently quenched,reheated, and then expanded by internal gas pressure to orient in theTD, and withdrawn, at a rate greater than that at which it is extruded,to stretch and orient it in the MD. Alternatively a flat film may beoriented by simultaneous or sequential stretching in each of twomutually perpendicular directions by means of a stenter, or by acombination of draw rolls and a stenter. A preferred oriented filmcomprises biaxially oriented polypropylene (known herein as BOPP), morepreferably the BOPP film described in EP 0202812.

[0096] The degree to which the film substrate is stretched depends tosome extent on the ultimate use for which the film is intended, but fora polypropylene film satisfactory tensile and other properties aregenerally developed when the film is stretched to between three and ten,preferably, seven or eight, times its original dimensions in each of TDand MD.

[0097] After stretching, the polymeric film substrate is normallyheat-set, while restrained against shrinkage or even maintained atconstant dimensions, at a temperature above the T_(g) of the polymer andbelow its melting point. The optimum heat-setting temperature canreadily be established by simple experimentation. Conveniently apolypropylene film is heat-set at temperatures in the range from about100° C. to about 160° C. Heat-setting may be effected by conventionaltechniques for example by means one or more of the following: a stentersystem; one or more heated rollers (e.g. as described in GB 1124886)and/or a constrained heat treatment (e.g. as described in EP 023776).

[0098] The film may comprise a major proportion of polypropylene such asisotactic polypropylene homopolymer, but also may comprise coextrudedmultilayer films where the polymer of at least one layer is isotacticpolypropylene homopolymer, and the polymer of one or both outer layersis a surface layer polymer having different properties to the isotacticpolypropylene homopolymer.

[0099] The sheet of the present invention may consist of only one layer,or the sheet may be multi-layered i.e. comprise a plurality of layers.The layers can be combined by lamination or co-extrusion. Preferably thesheet comprises at least three layers where at least one layer(s) aresandwiched between other layers such that none of such sandwichedlayer(s) form either surface of the sheet.

[0100] Thus for example a film of the invention may comprise a threelayer film where the polymer of a central or core layer comprises onepolymeric material. The core layer may have a thickness of about 90 toabout 98% of the total thickness of the film. The remainder of such athree layer film may comprise two outer layers of another polymericmaterial, with each outer layer having substantially identicalthickness.

[0101] Another film of the present invention may comprise a coextrudedfive layer film comprising a central core layer, two layers contiguousto the central core layer and two outermost layers, where the centralcore layer and such contiguous layers comprise one polymeric materialand the two outer layers comprise another polymeric material. Preferablythe central core layer has a thickness from about 70% to about 96%, morepreferably from about 76% to about 90%, of the total thickness of thefilm. Preferably each of such contiguous layers has substantially thesame thickness, which is more preferably from about 1% to about 6%, mostpreferably from about 1% to about 2%, of the total thickness of thefilm. Preferably each outer layer has substantially the same thickness,which is more preferably from about 1% to about 6%, most preferably fromabout 1% to about 2%, of the total thickness of the film.

[0102] A film of the invention may also be made by lamination of twocoextruded films

[0103] One or more layers of the films of the present invention may beopaque or transparent depending on the end use of the film. Such layersmay also comprise voids introduced by stretch orienting such a layercontaining spherical particles of a material higher melting than andimmiscible with the layer material (e.g. if the layer comprisesisotactic polypropylene homopolymer, then such particles may be,polybutylene terephthalate, as shown, for example, in U.S. Pat. No.4,632,869 and U.S. Pat. No. 4,720,716).

[0104] Multiple-layer films of the invention may be prepared in a rangeof thicknesses governed primarily by the ultimate application for whicha particular film is to be employed. For general use films, having amean thickness from about 2.5 μm to about 150 μm, preferably from about4 μm to about 100 μm are suitable. For certain applications, such aspackaging, preferred films have a mean thickness of from about 10 μm to50 μm, most preferably from about 8 μm to about 40 μm.

[0105] If desired, before coating a sheet of the present invention (e.g.with a gas barrier coating of the present invention and/or any othercoating and/or layer) may be subjected to a chemical or physicalsurface-modifying treatment to ensure that the coating and/or layer willbetter adhere to the sheet thereby reducing the possibility of thecoating peeling or being stripped from the sheet. Known prior arttechniques for surface pre-treatment prior to coating comprise, forexample: film chlorination, i.e., exposure of the film to gaseouschlorine; treatment with oxidising agents such as chromic acid, hot airor steam treatment; flame treatment and the like. A preferred treatment,because of its simplicity and effectiveness, is the so-called electronictreatment in which the sheet is passed between a pair of spacedelectrodes to expose the sheet surface to a high voltage electricalstress accompanied by corona discharge. Optionally if even adhesion ofthe coating is desired an intermediate continuous coating of a primermedium and/or anchor coating can be applied to a sheet surface treatedby any of the methods described herein. Primer materials may comprisetitanates and poly(ethylene imine) and may be applied as conventionalsolution coatings [such as poly(ethylene imine) applied as either anaqueous or organic solvent solution, e.g. in ethanol comprising about0.5 wt. % of the imine]. Another primer medium comprises theinterpolymerised condensation acrylic resins prepared in the presence ofa C₁₋₆alkanol as described in either: GB 1134876 (condensing amonoaldehyde with an interpolymer of acrylamide or methacrylamide withat least one other unsaturated monomer); or in GB 1174328 (condensing amonoaldehyde with acrylamide or methacrylamide, and subsequentlyinterpolymerising the condensation product with at least one otherunsaturated monomer).

[0106] In addition to the gas barrier coatings of the present inventionthat are described herein, one or more other layers may be applied to acoated sheet of the invention. Such other layers may convenientlycontain any of the additives and/or coatings conventionally employed inthe manufacture of sheets and such additives and/or coatings may beadded for more than one effect and/or for similar purposes. It is alsopossible that some of these other additives listed below may be directlyincorporated in the gas-barrier coatings of the present inventionprovided that such additives are not present in amounts which wouldsubstantially inhibit or destroy the gas barrier properties of thecoating. However it is generally preferred that the gas barrier coatingsof the invention are substantially free (preferably 90% free) of anyingredients other than the nano-particles and any optional gas barrierimproving ingredients.

[0107] Such additives and/or coatings may be selected from one or moreof the following ingredients, mixtures thereof and/or combinationsthereof, given below.

[0108] Additives may comprise: dyes; pigments, colorants; metallisedand/or pseudo-metallised coatings (e.g. aluminium); lubricants,anti-oxidants, surface-active agents, stiffening aids, gloss-improvers,prodegradants, UV attenuating materials (e.g. UV light stabilisers);sealability additives; tackifiers, anti-blocking agents, additives toimprove ink adhesion and/or printability, cross-linking agents (such asmelamine formaldehyde resin); adhesive layer (e.g. a pressure sensitiveadhesive); and/or an adhesive release layer (e.g. for use as the backingmaterial in the peel plate method for making labels).

[0109] Further additives comprise those to reduce coefficient offriction (COF) such as a terpolymer described in U.S. Pat. No. 3,753,769which comprises from about 2% to about 15% w/w of acrylic or methacrylicacid, from about 10% to about 80% w/w of methyl or ethyl acrylate, andfrom about 10% to about 80% w/w of methyl methacrylate, together withcolloidal silica and carnauba wax.

[0110] Still further additives comprise slip aids such as hot slip aidsor cold slip aids which improve the ability of a film to satisfactorilyslide across surfaces at about room temperature for examplemicro-crystalline wax. Preferably the wax is present in the coating inan amount from about 0.5% to about 5.0% w/w, more preferably from about1.5% to about 2.5% w/w. The wax particles may have an average sizeconveniently from about 0.1 μm to 0.6 μm, more conveniently from about0.12 μm to abut 0.30 μm.

[0111] Yet further additives comprise conventional inert particulateadditives, preferably having an average particle size of from about 0.2μm to about 4.5 μm, more preferably from about 0.7 μm to about 3.0 μm.Decreasing the particle size improves the gloss of the film. The amountof additive, preferably spherical, incorporated into the or each layeris desirably in excess of about 0.05%, preferably from about 0.1% toabout 0.5%, for example, about 0.15%, by weight. Suitable inertparticulate additives may comprise an inorganic or an organic additive,or a mixture of two or more such additives. Suitable particulateinorganic additives include inorganic fillers such as talc, andparticularly metal or metalloid oxides, such as alumina and silica.Solid or hollow, glass or ceramic micro-beads or micro-spheres may alsobe employed. A suitable organic additive comprises particles, preferablyspherical, of an acrylic and/or methacrylic resin comprising a polymeror copolymer of acrylic acid and/or methacrylic acid and/or a C₁₋₆esterthereof. Such resins may be cross-linked, for example by the inclusiontherein of a cross-linking agent, such as a methylatedmelamine-formaldehyde resin. Promotion of cross-linking may be assistedby the provision of appropriate functional groupings, such as hydroxy,carboxy and amido groupings, in the acrylic and/or methacrylic polymer.

[0112] Yet still further additives comprise fumed silica for the purposeof further reducing the tack of a coating at room temperature. The fumedsilica is composed of particles which are agglomerations of smallerparticles and which have an average particle size of, for example, fromabout 2 μm to about 9 μm, preferably from about 3 μm to about 5 μm, andis present in a coating in an amount, for example, from about 0.1% toabout 2.0% by weight, preferably about 0.2% to about 0.4% by weight.

[0113] It is contemplated that in some instances, the coatings used inthe present invention should have the additional benefit of aiding orenhancing the properties of other conventional additives and/or coatingsused on the same film. Thus two or more different coatings may exhibitthe same or similar properties.

[0114] Some or all of the desired additives listed above may be addedtogether as a composition to coat the sheet of the present inventionand/or form a new layer which may itself be coated (i.e. form one of theinner layers of a final multi-layered sheet) and/or may form the outeror surface layer of the sheet. Alternatively some or all of thepreceding additives may be added separately and/or incorporated directlyinto the bulk of the sheet optionally during and/or prior to the sheetformation (e.g. incorporated as part of the original polymer compositionby any suitable means for example compounding, blending and/orinjection) and thus may or may not form layers or coatings as such.These conventional other coatings and/or layers may thus be provided ontop of or underneath the gas barrier coatings of the present inventionand may be in direct contact thereto or be separated by one or moreother intermediate layers and/or coats.

[0115] If the film of the present invention comprises a polymer ifpossible it is preferred that conventional additives are added to thepolymer resin before the film is made instead of applying it as acoating or other layer. The incorporation of the blend into the resin isless expensive, less labour intensive, and more consistent than coatingsapplied to the surface of the film. If the additive is added to theresin, the mixing of the additives into the resin is done by mixing itinto molten polymer by commonly used techniques such as roll-milling,mixing in a Banbury type mixer, or mixing in an extruder barrel and thelike. The mixing time can be shortened by mixing the additives withunheated polymer particles so as to achieve substantially evendistribution of the agent in the mass of polymer, thereby reducing theamount of time needed for intensive mixing at molten temperature. Themost preferred method is to compound the additives with resin in atwin-screw extruder to form concentrates which are then blended with theresins of the film structure immediately prior to extrusion.

[0116] Formation of a film of the invention (optionally oriented andoptionally heat-set as described herein) which comprises one or moreadditional layers and/or coatings is conveniently effected by any of thelaminating or coating techniques well known to those skilled in the art.

[0117] For example a layer or coating can be applied to another baselayer by a coextrusion technique in which the polymeric components ofeach of the layers are coextruded into intimate contact while each isstill molten. Preferably, the coextrusion is effected from amulti-channel annular die such that the molten polymeric componentsconstituting the respective individual layers of the multi-layer filmmerge at their boundaries within the die to form a single compositestructure which is then extruded from a common die orifice in the formof a tubular extrudate.

[0118] A film of the invention may also be coated with one or more ofthe additives described herein using conventional coating techniquesfrom a solution or dispersion of the additive in a suitable solvent ordispersant. An aqueous latex, (for example prepared by polymerisingpolymer precursors of a polymeric additive) in an aqueous emulsion inthe presence of an appropriate emulsifying agent is a preferred mediumfrom which a polymeric additive or coating may be applied.

[0119] Coatings and/or layers (including those gas barrier coatings ofthe present invention) may be applied to either or both surfaces of thesheet. The or each coating and/or layer may be applied sequentially,simultaneously and/or subsequently to any or all other coatings and/orlayers. If a gas-barrier coating of the present invention is applied toonly one side of the sheet (which is preferred) other coatings and/orlayers may be applied either to the same side of the sheet and/or on thereverse (other) side of the sheet.

[0120] A coating composition may be applied to the treated surface ofsheet (such as the polymer film) in any suitable manner such as bygravure printing, roll coating, rod coating, dipping, spraying and/orusing a coating bar. Solvents, diluents and adjuvants may also be usedin these processes as desired. The excess liquid (e.g. aqueous solution)can be removed by any suitable means such as squeeze rolls, doctorknives and/or air knives. The coating composition will ordinarily beapplied in such an amount that there will be deposited following drying,a smooth, evenly distributed layer having a thickness of from about 0.02to about 10 microns, preferably from about 1 to about 5 microns. Ingeneral, the thickness of the applied coating is such that it issufficient to impart the desired characteristics to the substrate sheet.Once applied to the sheet a coating may be subsequently dried by hotair, radiant heat or by any other suitable means to provide a sheet ofthe present invention with the properties desired (such as an optionallyclear; optionally substantially water insoluble; highly oxygenimpermeable coated film useful, for example in the fields of packaging,labelling and/or graphic art).

[0121] It would also be possible to use combinations of more than one ofthe above methods of applying additives and/or components thereof to afilm. For example one or more additives may be incorporated into theresin prior to making the film and the one or more other additives maybe coated onto the film surface.

[0122] Other aspects of the present invention and preferred featuresthereof are also described in the claims.

[0123] The invention is now further illustrated by reference to thefollowing non-limiting Examples.

Nano-starch Coating Formulations

[0124] Formulation (i) Potato Starch

[0125] A nano-particle dispersion of extrusion modified potato starchcan be prepared as follows. A mixture of native potato starch (moisturecontent 18%) and glycerol (in a respective weight ratio of 21 to 4) wasblended in a mixer at room temperature until homogenous. The mixture wasfed at the rate of 8.4 kg per hour, into a twin screw extruder set at ascrew speed of 160 rpm. The extruder had nine zones with an adjustedtemperature profile of 20-80-90-100-100-100-100-100-95° C. An aqueoussolution of glyoxal (5% by weight) was injected into the fifth zone viaa piston dosing pump at a flow rate of 1.2 kg per hour, to give 1% ofpure glyoxal per weight of dry starch. The mixture was extruded througha five hole die (hole diameters 3 mm) and the extrudate granulated. Thegranules were cryogenically ground. The resultant powder was passedthrough a 100 to 200 micron polyester sieve to remove that particle sizefraction having too large a particle size, and 10 g of the fine powderwas mixed with 90 g of water and the mixture was shaken at roomtemperature to form a strong gel. The gel was allowed to collapse aftertime to form a homogeneous, transparent, light brown dispersion of lowviscosity, the average particle size of which was determined by dynamiclight scattering (DLS) to be 10 nm or larger.

[0126] Formulation (ii) Waxy Maize Corn Starch

[0127] A nano-particle dispersion of extrusion modified corn starchobtained from waxy maize may be prepared as described in method (i)above where the native potato starch is replaced by the same proportionof a corn starch from a waxy maize.

[0128] For the coatings used to prepare films Comp A to D, 10% of therelevant wax was also added to the relevant dispersion before it wasused to coat the film.

[0129] If necessary up to 0.2% of potassium sorbate can be added to anyof these starch dispersions as an anti-microbial agent and to increasethe storage stability of the dispersions by up to one month. Thisadditive does not significantly effect the OTR results generated herein.If the cryogenic grinding step is omitted the starch dispersion(otherwise prepared as described herein) is denoted as “raw” starch.

[0130] These nano-starch dispersions once prepared are used (preferablyimmediately) in the method described below to prepare the coated filmstested herein.

Coating Method

[0131] Coated films of the invention (Examples 1 to 19) and comparativeexamples Comp A to D were prepared by coating a base film with adispersion (freshly prepared as described above). Each of the filmsprepared and tested herein by the applicant have a pre-coat of aconventional primer applied (using yellow bar no 1—see Table 1) to thecorona treated surface of the base film so the subsequently appliedbarrier coatings would better adhere to the film surface.

[0132] Details of the film and coating used for each example are givenbelow and in the Tables. The films were coated using a model No K303 RKMulticoater with a bar coating head having a spiral wire around theoutside of the bar which acts to dispense a coating formulation onto asubstrate surface. Selecting different bars will alter the coatingthickness. The characteristics of the bars used to prepare the examplesherein are as follows. TABLE 1 (coating bars) Standard coating bar Wetfilm Used to prepare for K303 deposit Wire size examples No 1 (Yellow)0.00025″ = 0.003″ Primer only 6 micron (0.08 mm) No 2 (Red) 0.0005″ =0.006″ 6, 8, 9, 11¹, 12 micron (0.16 mm) 12¹, 13², 14² & Comp A to D No3 (Green) 0.001″ = 0.012″ 2, 3, 4, 7, 24 micron (0.31 mm) 13³, 14³, 15⁴,16 & 17 No 4 (Black) 0.0015″ = 0.020″ 1, 5 & 10, 40 micron (0.51 mm)

[0133] Footnotes

[0134] 1 Overcoated film—No 2 (red) bar used to draw down both thenano-starch coating and then the overcoat onto the film

[0135] 2 Overcoated film—No 2 (red) bar used to draw down the overcoatonto the film coated with nano-starch

[0136] 3 Overcoated film—No 3 (green) bar used to draw down thenano-starch coating onto the film.

[0137] 4 Overcoated film—No 3 (green) bar used to draw down both thenano-starch coating and then the overcoat onto the film

Prior Art Films

[0138] Various prior art films were made and tested by the applicant asdescribed herein, or data was used from published sources.

[0139] Comp A to D, and Examples 1 to 10; 13 to 15 and 17 to 19 herein,were prepared and tested by the applicant using various coatingcompositions on a base film of conventional clear BOPP film of 58 micronthickness and 18.9 m²kg⁻¹ yield (corona discharge treated on the side tobe coated) such as that available commercially from UCB Films under thetrade mark Rayoface C58. Examples 11, 12 and 16 herein were preparedusing an otherwise identical treated film of 50 micron thickness and22.0 m²kg⁻¹ yield available commercially from UCB Films under the trademark Rayoface C50.

[0140] It is believed that to a first order approximation using asdifferent substrates uncoated film of similar type (e.g. differentgrades of BOPP) and/or of different film thickness has little or noeffect on the oxygen barrier properties of the final film, which aredependent much more on the properties of the barrier coating used. Thusthe OTR data of Mobil 1 to 5 (25.4 micron thick film) and the PVdC film(32 micron) can be validly compared with the OTR data on C50 and C58base film generated herein by the applicant. The OTR value for theuncoated C50 and C58 base film used in the examples herein also given inTable 1 for comparison.

Mobil 1 to 5—Coatings of Silica Nano-particles in Binder Resin

[0141] Mobil 1 to Mobil 5 denotes those prior art BOPP films coated withnano-particles of organomontmorillonite clay in an ethylene acrylic acidcopolymer as binder resin, described and tested as described in WO00/40404 page 8 to 9, Table 2. Mobil 1 to 5 in Table 1 correspond toexamples SC96-034 to 038 respectively. For ease of comparison the datain Table 1 have been converted from the units quoted in the Mobilreference: i.e. coat weight from g/msi [≡g(in)⁻²×10⁻³] to gm⁻² and OTRfrom cc/100/in²/day/mil [≡cc(645.2 cm)⁻²(0.00254 cm)⁻¹day⁻¹] to(cm)³m⁻²day⁻¹. The OTR data in Mobil 1 to 5 were stated to have beennormalised for a standard BOPP film of 25.4 micron thickness.

Conventional PVdC Coated Film

[0142] For comparison the table also includes OTR data for aconventional barrier coated film of clear BOPP of 32 micron thicknessand 32.5 m²kg⁻¹ yield and coated on both sides with PVdC. This is thefilm available commercially from UCB Films under the trade designationRX32.

Comp 1 to 6—Coatings of Micron Sized Starch

[0143] Comp 1 is a C58 base film coated with an oxypropylated maizestarch (available from Amylum) which has been extruded afterderivatisation to form starch particles of average size above onemicron.

[0144] Comp 2 is a C58 base film coated with a conventional gum arabicparticles of average size above one micron and available commerciallyfrom Aldrich Chemicals.

[0145] Comp 3 is a C58 base film coated with a potato starch ofconventional micron particle size, the starch being that used to prepareFormulation (i) herein, but before its treatment by thethermo-mechanical process as described herein to form nano-sizedparticles.

[0146] Comp 4 is a C58 base film coated with a modified waxy maize cornstarch (available from Amylum) cross-linked prior to extrusion to formstarch particles of average size above one micron.

[0147] Comp 5 is a C58 base film coated with a maize starch which issoluble cold water and of conventional micron particle size (less than20% of the particles being above 100 microns) and is availablecommercially from Roquette under the trade name Pregeflo.

[0148] Comp 6 is a C58 base film coated with the starch used in Comp 1together with 15% by weight in the coating of as a cross-linker theglyoxal available commercially from Clairant under the trade nameCartabond TSI.

Comp A to D—Coatings of Nano-sized Starch with 10% Wax

[0149] Comp A is a C58 base film coated as described herein (withreference to Tables 1 and 2 herein) with the cryogenically ground waxymaize corn starch of Formulation (ii) described herein (90% by dryweight) together with (10% by dry weight) of a conventional polyethylenewax (such as that available commercially from Omya).

[0150] Comp B is a C58 base film coated with the composition of Comp Awhere the polyethylene wax was replaced by the same amount of a carnaubawax available commercially from Omya under the trade name MichemiubeML160.

[0151] Comp C is a C58 base film coated with the composition of Comp Awhere the polyethylene wax was replaced by the same amount of a montanwax available commercially from Clariant under the trade designation KPSemulsified with a suitable surfactant.

[0152] Comp D is a C58 base film coated with the composition of Comp Awhere the polyethylene wax was replaced by the same amount of a paraffinwax.

Test Results

[0153] The oxygen transfer rate (OTR) for each film at varioustemperatures and relative humidities (RH) are given in Table 2 below.When tested by the applicant the test method used to measure OTR wasthat standard method described in ASTM D1432 (with, where necessary thetemperature and RH of the test conditions changed as specified in Table2). TABLE 2 Prior Art Films Oxygen transfer rate Coat [OTR] in (cm)³m⁻²(24 hr)⁻¹ Example weight in 23° C., 23° C., 23° C., (bar) gm⁻² 0% RH 50%RH 70% RH Uncoated OPP films. C50 No coat 1,039.97 No data No data C58No coat 1,041.01 No data No data PVdC coated OPP film (RX32) PVdC ≈2.515 No data 20 Silica nano-particles in binder resin Mobil 1 0.78 1473 Nodata No data Mobil 2 0.89 1209 No data No data Mobil 3 1.04 713 No dataNo data Mobil 4 1.28 434 No data No data Mobil 5 1.42 140 No data Nodata Micron sized starch Comp 1 ≈1 89.76 No data 380.89 Comp 2 ≈1 54.94No data 557.47 Comp 3 ≈1 164.01 No data 650.31 Comp 4 ≈1 250.29 No data630.48 Comp 5 ≈1 100.18 No data 470.08 Comp 6 ≈1 79.77 No data 538.97Nano-starch plus 10% wax Comp A ≈1.8 40.39 132.87 413.47 Comp B ≈1.855.13 81.24 372.99 Comp C ≈1.8 145.33 174.13 550 Comp D ≈1.8 33.15 60.95495.31

Films Coated with Nano-starch

[0154] Various films of the present invention were made and tested bythe applicant as described herein.

EXAMPLES 1 to 5 Coatings of Nano-sized Potato Starch

[0155] Examples 1 to 3 are C58 base film coated as described herein(with reference to Tables 1 and 3 herein) with the cryogenically groundpotato starch of Formulation (i) described herein.

[0156] Examples 4 and 5 are C58 base film coated as described herein(with reference to Tables 1 and 3 herein) with the raw potato starch ofFormulation (i) described herein.

EXAMPLES 6 to 10 Coatings of Nano-sized Waxy Maize Corn Starch

[0157] Examples 6 to 10 are C58 base film coated as described herein(with reference to Tables 1 and 3 herein) with the cryogenically groundwaxy maize corn starch of Formulation (ii) described herein; exceptExample 9 used this starch raw (i.e. not cryogenically ground).

EXAMPLES 11 to 15 Overcoated Nano-starch Coatings

[0158] Example 11 is C58 base film coated as described herein (withreference to Tables 1 and 3 herein) with the cryogenically ground waxymaize corn starch of Formulation (ii) described herein, and then furthercoated at a coat weight of ≈1 gm⁻² with an overcoat of an aqueousdispersion comprising 15% by weight of the acrylic copolymers availablecommercially from UCB Chemicals under the trade designation WB 1240.

[0159] Example 12 is C58 base film coated as described herein (withreference to Tables 1 and 3 herein) with the cryogenically ground waxymaize corn starch of Formulation (ii) described herein, and then furthercoated at a coat weight of ≈1 gm⁻² with an overcoat of an aqueousdispersion comprising 25% by weight of the polyester availablecommercially from SK Chemicals or UCB Chemicals under the tradedesignation EW110.

[0160] Example 13 is C58 base film coated as described herein (withreference to Tables 1 and 3 herein) with the cryogenically ground potatostarch of Formulation (i) described herein, and then further coated at acoat weight of ≈1 gm⁻² with an overcoat of an aqueous dispersioncomprising 15% by weight of the polyester available commercially from SKChemicals or UCB Chemicals under the trade designation EW100.

[0161] Example 14 is C50 base film coated as described herein (withreference to Tables 1 and 3 herein) with the cryogenically ground potatostarch of Formulation (i) described herein, and then further coated at acoat weight of ≈1 gm⁻² with an overcoat of an aqueous dispersioncomprising 15% by weight of the acrylic copolymers availablecommercially from UCB Chemicals under the trade designation WB 1240.

[0162] Example 15 is C58 base film coated as described herein (withreference to Tables 1 and 3 herein) with the cryogenically ground potatostarch of Formulation (i) described herein, and then further coated at acoat weight of ≈1 gm⁻² with an overcoat of an aqueous dispersioncomprising 27% by weight of the polyester available commercially from SKChemicals or UCB Chemicals under the trade designation EW100.

Test Results

[0163] The oxygen transfer rate (OTR) for each film at varioustemperatures and relative humidities (RH) are given in Table 3 below.Compared to the prior art films it can be seen that the OTR values ofnano starch coated films of the invention in the absence of binderresins are much lower (i.e. coatings of the invention are much betteroxygen barriers) than either the prior art conventional PVdC coatedbarrier film, the Mobil films which use inorganic nano-particles in athermoplastic polymer binder or films coated with micron sized starch.TABLE 3 Films of the invention - nano starch Coat OTR/(cm)³m⁻² day⁻¹weight/ 23° C. 23° C. 23° C. 23° C. Example gm⁻² 0% RH 30% RH 50% RH 70%RH Nano-starch (potato starch) 1 ≈2 4.29 10.9 15.52 No data 2 ≈1.7 2.815.4 21.63 No data 3 2.2 to 0.66 No data 24.35 418.84 2.4 4 1.9 1.8 Nodata 31.54 442.59 5 3.9 0.84 No data 12.41 166.68 Nano-starch (waxymaize corn starch) 6 ≈1.4 3.85 11.93 18.25 No data 7 ≈1.7 1.5 12.09 7.519.52 8 1.8 1.74 No data 32.76 379.71 9 ≈1.7 2.43 No data 36.21 393.1510 6.5 4.44 No data 40.44 411.81 Nano starch plus overcoat 11 1.4 2.2913.97 18.76 27.1 12 1.4 1.46 9.64 16.11 27.05 13 2.22 2.9 No data 7.79364.16 14 2.06 2.04 No data 19.24 428.08 15 2.32 1.65 No data 15.69173.24

Films Coated with Starch Plus Sucrose Additive

[0164] Various other films of the present invention were made and testedby the applicant as described herein.

EXAMPLES 16 to 17 Coatings of Nano-sized Starch with Sucrose

[0165] Example 16 is a C50 base film coated as described herein (withreference to Tables 1 and 4 herein) with the cryogenically ground potatostarch of Formulation (i) described herein to which sucrose has beenadded in a weight ratio of dry ingredients of 9 to 1 (starch tosucrose).

[0166] Example 17 is a C50 base film coated as described herein (withreference to Tables 1 and 4 herein) with the cryogenically ground potatostarch of Formulation (i) described herein to which sucrose has beenadded in a weight ratio of dry ingredients of 7 to 3 (starch tosucrose).

EXAMPLES 18 to 19 Coatings of Micron-sized Starch with Sucrose

[0167] Example 18 is a C58 base film coated as described herein (withreference to Tables 1 and 4 herein) with an oxypropylated maize starchwhich has been extruded after derivatisation which forms starchparticles of average size above one micron (that used in Comp 1 herein)to which sucrose has been added in amount of 30% by weight of dryingredients.

[0168] Example 19 is a C58 base film coated as described herein (withreference to Tables 1 and 4 herein) with the micron sized starch of Comp1 herein, together with 15% of a glyoxal cross-linker (that availablecommercially from Clariant under the trade name Cartabond TSI) and 30%of sucrose (amounts by weight of dry ingredients).

Test Results

[0169] The oxygen transfer rate (OTR) for each film at varioustemperatures and relative humidities (RH) are given in Table 4 below. Asa further aspect of the invention it can be seen that adding sucrose(even to a coating of conventional micron-sized starch) leads to agreatly enhanced oxygen barrier (compared with OTR data for themicron-sized starch coatings Comp 1 to 6 in Table 2). TABLE 4 Films ofthe invention - sucrose additive Coat OTR/(cm)³m⁻² day⁻¹ weight/ 23° C.23° C. 23° C. 23° C. Example gm⁻² 0% RH 30% RH 50% RH 70% RH Nano starchplus sucrose 16 ≈2.5 0.31 No data 18.68 593.31 17 ≈2.5 0.36 No data 3.85582.89 Micron-sized starch plus sucrose 18 ≈1 23.03 No data No data424.17 19 ≈1 22.62 No data No data 469.63

1 A substantially planar self-supporting sheet comprising on at leastone side thereof a substantially continuous adherent coating thereon tocreate a barrier to oxygen, the coating comprising nano-particles, thesheet characterised in that: (a) the nano-particles form a substantiallycontinuous phase on the sheet; and/or (b) the nano-particles are presentin an amount of at least about 20% by dry weight of the coating; and/or(c) the coating is substantially free of polymeric binder, synthetichydrophilic resin and/or a resin with a hydrophilic/hydrophobiccharacter. 2 A coated sheet as claimed in claim 1, in which the coatingfurther comprises, in an oxygen barrier improving amount, a oxygenbarrier improving component which acts to enhance the oxygen barrierproperties of the nano-particle coat. 3 A substantially planar selfsupporting sheet comprising on at least one side thereof a substantiallycontinuous adherent coating thereon to create a barrier to oxygen, thecoating comprising nano and/or micro-particles, the sheet characterisedin that: the coating further comprises, in an oxygen barrier improvingamount, a oxygen barrier improving component which acts to enhance theoxygen barrier properties of the particulate coat.
 4. A coated sheet asclaimed in claim 3, in which the particles have a mean particle sizeless than about 500 nm.
 5. A coated sheet as claimed in any precedingclaim, in which the particles have a mean particle size less than about300 nm.
 6. A coated sheet as claimed in claim 5, in which the particleshave a mean particle size from about 5 nm to about 300 nm.
 7. A coatedsheet as claimed in claim 6, in which the particles have a mean particlesize from about 10 nm to about 200 nm.
 8. A coated sheet as claimed inclaim 7, in which the particles have a mean particle size from about 10nm to about 100 nm.
 9. A coated sheet as claimed in any preceding claim,in which the particles comprise a biopolymer.
 10. A coated sheet asclaimed claim 9, in which the biopolymer is selected from one or morecarbohydrate; polysaccharide; cereal protein, vegetable protein, animalprotein; hydro-colloid; effective modified derivatives thereof; and/oreffective mixtures thereof.
 11. A coated sheet as claimed claim 10, inwhich the biopolymer is selected from starch, cellulose, glycogen,hemi-cellulose, chitin, fructan, inulin, lignin, pectic substances, gum,gluten, whey protein, gelatin, modified derivatives thereof; and/ormixtures thereof.
 12. A coated sheet as claimed claim 11, in which thebiopolymer is selected from native and/or modified starch obtainedand/or obtainable from plant(s).
 13. A coated sheet as claimed claim 12,in which the biopolymer is selected from a starch, starch-ether,starch-ester and/or oxidised starch obtained and/or obtainable from oneor more root(s), tuber(s) and/or cereal(s).
 14. A coated sheet asclaimed in any of claims 9 to 13, in which the biopolymer particles areobtained and/or obtainable from a mechanical thermoplastic processcomprising a cross-linking step using a cross-linking agent.
 15. Acoated sheet as claimed in claim 14, in which the cross-linking agent isa polyaldehyde, optionally glyoxal.
 16. A coated sheet as claimed in anypreceding claim, having an oxygen transfer rate (OTR) measured at 23° C.and 0% relative humidity of less than about 10 cm³/m²/day,
 17. A coatedsheet as claimed in claim 16, having an OTR of less than about 5cm³/m²/day.
 18. A coated sheet as claimed in claim 17, having an OTR ofless than about 3 cm³/m²/day.
 19. A coated sheet as claimed in anypreceding claim, which is 95% free of a oxygen barrier inhibitingcomponent which comprises one or more organic compound and/or polymerwhich comprises linear carbon chains of greater than 10 carbon atoms.20. A coated sheet as claimed in claim 19, in which the coating is freeof an oxygen barrier inhibiting amount of a natural or synthetic wax.21. A coated sheet as claimed in any of claims 2 to 20, in which the gasbarrier improving component has one or more of the following selectedproperties: small polymers and/or oligomers having a short chain lengthsuch than the mean number of repeat units on the oligomer and/or polymerchain is less than or equal to about 20; mean molecular weight (M_(W))less than about 1000; chemical and/or physical compatibility with theparticles in the coat to aid close packing; components havingcombinations more than one of these properties in the same component;and/or mixtures of different components with at least one of theseproperties.
 22. A coated sheet as claimed in any of claims 2 to 21, inwhich the gas barrier improving component comprises one or moreconstituent monomers dimers, trimers and/or oligomers from whichbiopolymers may be made.
 23. A coated sheet as claimed in claim 22, inwhich the gas barrier improving component is selected from one or more:dextrin (optionally maltodextrin and/or cyciodextrin); low molecularweight hydroxy compound (optionally glycerol); low molecular weightcarbohydrates (optionally saccharides and/or sugars) and/or anyeffective mixtures and/or combinations thereof.
 24. A coated sheet asclaimed in any of claims 2 to 23, in which the barrier improvingcomponent is present in a total amount from about 10% to about 99% byweight of the coating.
 25. A coated sheet as claimed in claim 24, inwhich the gas barrier improving component is present in a total amountfrom about 20% to about 95% by weight of the coating.
 26. A coated sheetas claimed in claim 25, in which the barrier improving component ispresent in a total amount from about 40% to about 90% by weight of thecoating.
 27. A coated sheet according to any preceding claim, in whichthe sheet comprises a cellulosic material, polymeric material and/orthermoplastic polymer,
 28. A coated sheet according to claim 27, inwhich the sheet comprises a polyolefin, polyurethane, polyester,polyamides and/or non-hydrocarbon polymer and which is optionallyoriented in at least one direction.
 29. A coated sheet substantially asdescribed herein with reference to the Examples 1 to
 19. 30. Acomposition suitable for applying as a coating to at least one side of asuitable prepared substantially planar self supporting sheet, thecomposition comprising a coating as represented in any preceding claim.31. A coating composition substantially as described herein withreference to the Examples 1 to
 19. 32. A method for coating at least oneside of a substantially planar self supporting sheet, the methodcomprising the steps of: a) optionally treating the sheet surface(optionally by primer coat and/or corona discharge) to better receive acoating; b) preparing a coating formulation as claimed in either claim30 or 31, c) applying and fixing said formulation to at least onesurface of the sheet to form a coating thereon.
 33. A coated sheetobtained and/or obtainable by the method claimed in claim
 32. 34.Packaging for an article, the packaging comprising a coated sheet asclaimed in any of claims 1 to 29 or
 33. 35. An article packaged withpackaging as claimed in claim
 34. 36. A label and/or graphic art displaycomprising a coated sheet as claimed in any of claims 1 to 29 and/or 33.37. An article comprising a label and/or graphic art display as claimedin claim
 36. 38. Use of a composition as claimed in either claims 30 or31, for the purpose of coating a sheet to provide at least an improvedoxygen barrier to said sheet.
 39. Use of an barrier improving agent asrepresented in any of claims 21 to 26, in a coating for the purpose ofimparting improved oxygen barrier properties to a sheet.